Assessment of orange spotting severity in oil palm using multi spectral reflectance approach

Orange Spotting (OS) caused by a variant of Cadang-Cadang Coconut Viroid (CCCVd) is an emerging problem in oil palm plantations of Malaysia. This work was an effort to study epidemiology of OS in an oil palm plantation using Precision Agriculture (PA) tools. A 4.2 ha plot were established in a commercial plantation situated at Sungai Buloh, Selangor. A total of 587 geo-referenced mature trees were visually observed to identify OS-infected oil palm trees based on symptoms. OS incidence and severity data were acquired from all 587 observed oil palm trees and the data subjected to a series of spatial variability analysis. Forty symptomatic oil palm trees were systematically selected based on OS severity for leaflets sampling. Leaflets from another 10 nonsymptomatic oil palm trees were also sampled. The sampled leaflets were subjected to spectral reflectance acquisition followed by Dot-Blot analysis. A second study plot was established in a mature oil palm stand (approximately 10 km away from the first study site) which was selected based on presence of potassium deficiency and absence of OS. The sampled leaflets were subjected to spectral reflectance acquisition followed by leaf Potassium (K) analysis. The incidence of OS incidence in the study area was 74.3%. OS severity ranged from 0-92.3%. The spatial structure of OS severity was described by an exponential model with an effective range of 29.1 m. Orange spotting severity exhibited a strong spatial dependence with a nugget to sill ratio of 0.15. The spatial variability map of OS severity revealed spatial clustering of kriged values, where 73% of the study area showed low severity (1-30%), 25% showed moderate severity (30-60%) and approximately 2% showed high severity (>60%). This study demonstrates the utility of geo-spatial information in understanding the OS severity scale which could assist in site-specific disease monitoring and intervention. Dot-blot assay showed CCCVd presence in 40% of the samples obtained from nonsymptomatic oil palm trees. Symptomatic leaf samples showed a significant correlation (r=-0.70) between leaf chlorophyll reflectance and OS severity. Spectral reflectance of symptomatic leaves was significantly lower than that of nonsymptomatic leaves at the 465-711 nm wavelength regions. Spectral reflectance of healthy and asymptomatic leaves, however, did not exhibit significant differences across all wavelengths investigated. In symptomatic leaves, spectral reflectance showed a decreasing trend with an increase in OS severity of up to 60% at the 555 nm and 780-1000 nm wavelengths. Among the vegetation indices tested, MCARI1 and mSR705 performed best in predicting OS severity with a goodness of fit (measured vs. predicted) of 66% and 56%, respectively. Reflectance between OS-infected and K-deficient leaves showed significant separability (p < 0.05) at 400-538 nm and 667-688 nm wavelengths. Reflectance of K-deficient leaves was significantly different than that of OS-infected leaves from OS severity classes (1-20%, 21-40%, 41-60% and 61-80%). All oil palm leaflets exhibited green peak at 555 nm wavelength, with average reflectance value of 0.15.

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